A Tunable Coaxial Filter with Minimum Variations in Absolute Bandwidth and Q using a Single Tuning Element

This paper presents a novel high Q tunable coaxial filter, which maintains a constant absolute bandwidth and a constant Q over the tuning range. The key feature of the proposed filter configuration is that it is tuned by a single rotational mechanism irrespective of the filter order. The filter is capable to achieve a tuning range of 30% with a very minimum variations in bandwidth and insertion loss. A prototype 4-pole filter is developed at 2.5 GHz with a fractional bandwidth of 4% to verify the concept. The measured tuning range of the filter is 20% with insertion loss better than 0.4 dB over the entire tuning range. The measured bandwidth variation is within +/- 10% and insertion loss variation is within 0.05 dB over the tuning range. The concept is easily expandable to filters with higher order.